Electron emitters



Dec. 10, 1963 R. STRATTON 3,114,070

ELECTRON EMITTERS Filed Dec. 12, 1958 2 Sheets-Sheet 1 HTTOFPNEY Dec. 10, 1963 R. STRATTON 3,114,070

ELECTRON EMITTERS Filed Dec. 12, 1958 2 Sheets-Sheet 2 PULSE GEN.

22 Z/ Z5 Z8 25 27 26 I R05 PT 67 TON HTTOP/VE) United States Patent 3,114,070 ELECTRON EMITTERS Robert Stratton, Ashton-on-Mersey, Sale, England, as-

signs:- to Associated Electrical industries (Manchester) Limited, a British company Filed Dec. 12, 1958, Ser. No. 779,953 Claims priority, application Great Britain Dec. 16, 1957 2 Qlairns. (ill. 313346) This invention relates to electron emitters for emitting electrons into an evacuated space.

The thermionic electron emission current from the surface of a solid into an evacuated space depends upon (a) the velocity of the electrons in the solid, i.e. the energy and (b) the surface work function, i.e. the retarding potential barrier at the surface of the solid which attracts an escaping electron.

As is well known electron emitters, as used hitherto, have required either a heater, in the case of thermionic cathodes, or alternatively in the case of photo electric cells, they have required activation from an external source of energy.

In order to obtain a high electron emission it is normal ractice to coat the solid with a material with a low surface work function, e.g. an oxide of calcium, barium or strontium in the case of thermionic valves and say caesium in the case of photo electric cells.

The main object of the invention is to provide an improved electron emitter of the kind referred to which will emit electrons into an evacuated space without requiring either a heater or external activating rays.

According to the present invention an electron emitter comprises a body of covalent semi-conductor material having a surface bordering an evacuated space and means for applying a pulsed high voltage across said semi-conductor body to cause electron emission into the evacuated space.

The semi-conductor body is preferably in the form of a fiat block or disc or it may for example be in the form of a hollow cylinder.

The high voltage is pulsed so as to obtain a strong electric field without over-heating the semi-conductor material.

The effect should be most pronounced in semi-conducting materials where the interaction between the electron and the lattice thermal vibrations is by non-polar modes, that is to say, semi-conductors in which the electronlattice coupling is weak. Examples of these are the elements and compounds of the elements of group IV of the periodic table such as germanium and silicon.

By applying a strong electric field to such a material the electron energy can be raised to a value corresponding to a cathode temperature many times the melting point of the cathode material. Therefore, since the emission current is exponentially related to the energy, it can, by this means, be increased to a large amount.

it is well known that when an electric field is applied to any electronic conductor both energy and momentum are supplied to the free electron gas. Thus the kinetic energy of the electrons will rise above its static equilibrium value while the electron gas as a whole will start to drift in the direction of the field, i.e. a current flows. The electron gas in turn supplies energy and momentum to the crystal lattice by collisions between the electrons and the thermal lattice vibrations, the former corresponding to the well known joule heating of the conductor.

In a steady state, the electron gas achieves a mean kinetic energy and momentum, for which the ingoing and outgoing rates of change of energy and momentum are exactly balanced. The mean kinetic energy of the electron in the steady state can be described by an elfective electron temperature T which depends on the applied field and is in excess of the lattice temperature T For low applied fields, T is only slightly greater than T and the drift velocity is proportional to the field, i.e. Ohrns law is obeyed. However, when the applied field is sufficiently high, T will be considerably greater than T and indeed greater than the melting point of the crystal lattice. This effect is associated with a decrease in the ratio of the drift velocity to the applied field, i.e. the mobility. In order to achieve this hot electron effect, fields are required which, if applied for any length of time, would cause excessive heating of the lattice causing disruption. It is thus necessary to use pulsed fields which are not of sufficient duration to cause overheating. Some moderate heating of the lattice would be advantageous as this would lead to increased electron densities.

While this effect will occur in all electronic conductors, the highest electron temperatures, for a given field, will be achieved with covalent semiconductors, e.g. Ge, Si, SiC, etc. due to the relatively weak interaction between electrons and thermal lattice vibrations, i.e. high mobilities.

The maximum electric field applied must be less than the dielectric breakdown field of the particular material used. It will thus be necessary to use semi-conductors with a sufficiently large forbidden energy gap (the separation in energy between the bottom of the conduction band and the top of the valence band) to allow for high electron energies before avalanche breakdown sets in.

It may be noted that some degree of electro-multiplication by impact ionisation may be permissible and indeed advantageous in increasing the number of available electrons, provided this does not lead to avalanche breakdown.

The production of high electron temperatures and possibly increased electron densities will lead to very large thermionic emission currents from the free surface of a semi-conducting cathode far in excess of those which could be achieved by conventional heating of the cathode as a whole.

Further improvement is possible by coating the semiconducting cathode with a substance of low work function, for example, caesium.

In order that the invention may be more clearly understood reference will now be made to the accompanying drawings in which FIGS. 1-4 show four examples of apparatus embodying the invention.

The device shown in FIG. 1 essentially comprises a base plate l which is of conducting material, e.g. copper, and supports a second plate 2 by means of a hollow insulating cylinder 3. From the plate 2 there extends a rod 4 carrying at the end an anode disc 5. A disc of semi-conductor material 6 is mounted on an insulating block 7 which in turn is carried on the base plate 1. Conductor strips 8 and 9 extend from opposite edges of the semi-conductor 6 to the copper ring electrode 10 and terminal 1 respectively. These terminals are connected to a pulse generator, indicated by the rectangle 12. At the same time a high voltage D.C. potential is applied between the anode and cathode from a high voltage source, indicated diagrammatically at 13.

FIG. 2 shows an alternative arrangement in which a semi-conductor device is in the form of a cylindrical diode. A rod of semi-conductor 15 is mounted between terminal blocks 16 and 17 carried on disc insulators 18 which discs are contained in the conducting cylinder Zll.

H6. 3 shows a further embodiment in which the semiconductor material is in the form of a hollow cylinder 21. Voltage pulses are applied across it by a pulse generator 12 which is connected to the conducting rings 22 and 23, the latter being elongated. The anode disc 5 is in this case located within the elongated ring 23. The

3,1 3 rod carrying the anode 5 may be supported in any suitable manner and is shown as carried by an insulating plate 24 extending across the end of the ring 13.

FIG. 4 shows a device or" the kind shown in FIG. 3 but fitted with accelerating electrodes. These are in the form of rings 25, 26 seoarated longitudinally from each other and from the ring 23 by insulating rings 2? and 23 respectively, as shown in the drawing.

Suitable accelerating potentials are applied to the rings 23 and 24 by batteries, which potentials are intermediate between that of the ring 23 and anode S. In all cases the interior or" the device would be evacuated.

The arrangements of FIGS. 1 and 2 each form a suitable internal space which can be evacuated whilst in the arrangements of FIGS. 3 and 4 the various electrodes and insulators would be sealed together end to end to form a hollow cylinder, one end of which is sealed by the insulator 24 and the other end of which is sealed by a disc 29 extending across the interior of the ring 23 and preferably of insulating material, though it may be conductive.

Clearly nurn rous other constructions may be envisaged and it will be appreciated that in each case the device may be either initially evacuated and sealed or alternatively continuously evacuated.

What I claim is:

1. An electron emitter comprising a body of semiconductor material, means providing an evacuated space more with a surface of said body bordering thereon, means supplying a pulsed high electrical voltage across said semi-conductor surface lessthan the dielectric breakdown for the semi-conductor material and below avalanche breakdown of the semi-conductor for producing electron emission into the evacuated space, and an anode in said evacuated space for receiving the electrons.

2. In an electron discharge tube, means enclosing an evacuated space, a body of semi-conductor material with a surface thereof exposed to said evacuated space, means supplying a pulsed high electrical voltage across said semi-conductor surface less than the dielectric breakdown for the semi-conductor material and below avalanche breakdown of the semi-conductor for producing electron emission in the evacuated space, an anode in said evacuated space for receiving the electrons, and means includin an accelerating electrode located intermediate the electron emitting surface of the semi-conductor and said anode and applying a voltage thereto intermediate the electrical potentials of said electron emitting surface and said anode.

References Cited in the file of this patent UNITED STATES PATENTS 2,501,089 Pomerantz Mar. 21, 1950 2,578,754 Smits Dec. 18, 1951 2,960,659 Burton Nov. 15, 1960 3,029,359 White Apr. 10, 1962 

1. AN ELECTRON EMITTER COMPRISING A BODY OF SEMICONDUCTOR MATERIAL, MEANS PROVIDING AN EVACUATED SPACE WITH A SURFACE OF SAID BODY BORDERING THEREON, MEANS SUPPLYING A PULSED HIGH ELECTRICAL VOLTAGE ACROSS SAID SEMI-CONDUCTOR SURFACE LESS THAN THE DIELECTRIC BREAKDOWN FOR THE SEMI-CONDUCTOR MATERIAL AND BELOW AVALANCHE BREAKDOWN OF THE SEMI-CONDUCTOR FOR PRODUCING ELECTRON EMISSION INTO THE EVACUATED SPACE, AND AN ANODE IN SAID EVACUATED SPACE FOR RECEIVING THE ELECTRONS. 